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Instrumentation

Percent Span Explained

Percent span expresses a process value as a percentage of the instrument range (LRV to URV). Learn the formula, the difference from 4-20 mA scaling, and common mistakes.

TypeEngineering guide — concept explainer

Definition

Percent span (also called percent of range) expresses a process value as a percentage of the instrument's configured range. The range is defined by a lower range value (LRV) and an upper range value (URV). At LRV, percent span is 0%. At URV, percent span is 100%. Percent span is a linear interpolation between these two endpoints.

Why it matters

Instrument technicians and control engineers routinely need to convert between a process value in engineering units (bar, °C, L/s) and its percent-of-span equivalent. Calibration checks, alarm setpoint verification, and control loop tuning all involve this conversion. Percent span is distinct from the 4-20 mA signal — percent span describes where the process value sits within the instrument range, while 4-20 mA describes the electrical signal representing that position.

Formula

Percent span from process value
% span = ((PV - LRV) / (URV - LRV)) x 100
Process value from percent span
PV = LRV + (% span / 100) x (URV - LRV)
Span
Span = URV - LRV

Units involved

  • •PV — process value in engineering units (bar, °C, L/s, etc.)
  • •LRV — lower range value (the PV at 0% span)
  • •URV — upper range value (the PV at 100% span)
  • •Span — the difference URV - LRV, in the same units as PV
  • •% span — the position of PV within the range, from 0% to 100%

Concept diagram

LRV0 barURV100 barPV = 65 barSpan = URV - LRV = 100 bar65% of span

Worked example

A pressure transmitter is ranged 0-100 bar (LRV = 0, URV = 100). The current process value is 65 bar. What is the percent span?

  1. 01LRV = 0 bar, URV = 100 bar, PV = 65 bar
  2. 02% span = ((PV - LRV) / (URV - LRV)) x 100
  3. 03% span = ((65 - 0) / (100 - 0)) x 100
  4. 04% span = (65 / 100) x 100
  5. 05% span = 65%
Result

65 bar on a 0-100 bar range is 65% of span.

Common mistakes

  • •Confusing percent span with 4-20 mA. Percent span is a dimensionless ratio describing position within the range. The 4-20 mA signal is an electrical representation of that same position but uses a different formula (offset by 4 mA and scaled over 16 mA). Use the 4-20 mA Scaling calculator for signal-to-PV conversions.
  • •Forgetting that LRV is not always zero. A range of 50-250 °C has LRV = 50, not zero. At PV = 50 °C, percent span is 0%, not 20%.
  • •Applying percent span outside the linear range. This formula assumes a linear relationship between PV and percent span. Non-linear instruments (e.g., with square-root extraction for DP flow) require additional conversion.
  • •Mixing up span and URV. Span is URV minus LRV, not URV itself. For a range of 10-50 bar: URV = 50, but span = 40.

When to use the calculator

Use the Sensor Percent Span calculator to convert between a process value and its percent-of-span equivalent. Enter the LRV, URV, and either the PV or the percent span to find the other. This is useful for calibration checks, alarm setpoint verification, and quick field calculations.

Sensor Percent Span Calculator →4-20 mA Scaling Calculator →

FAQ

What is the difference between percent span and 4-20 mA?
Percent span and 4-20 mA both describe position within a range, but percent span is dimensionless (0-100%) while 4-20 mA is an electrical signal. At 0% span the signal is 4 mA; at 100% span the signal is 20 mA. They are related but use different formulas. ProcessConvert has separate calculators for each: the Sensor Percent Span calculator and the 4-20 mA Scaling calculator.
Can percent span go below 0% or above 100%?
Mathematically, yes — if PV is below LRV, percent span is negative; if PV is above URV, percent span exceeds 100%. In practice, this means the process value is outside the instrument range. Whether the instrument can read or transmit beyond its range depends on the transmitter model and configuration.
How do I handle a suppressed or elevated zero?
A suppressed zero means LRV > 0 (e.g., 50-250 °C). An elevated zero means LRV < 0 (e.g., -40 to 60 °C). The formula is the same in both cases: % span = ((PV - LRV) / (URV - LRV)) x 100. The calculator handles any LRV and URV values.
Is this the same as percent of full scale?
Only if LRV = 0. Percent of full scale refers to the ratio PV / full-scale value. Percent of span refers to the position within the configured range (LRV to URV). When LRV is not zero, the two differ. In instrumentation, percent of span is the standard term.